Those could be the recommended screensize for readable text, with a small border of 2x8 pix horizontal and a 2x23pix vertical border.

PDFLRF makes LRF based on images and it also uses 584 width x 754 height as the final output. So I don't think the resolution is only for text file. Of course, I am assuming that image based LRFs are displayed in the same resolution as image files.